1. Field of the Invention
This invention relates to a method for suppressing vibration of an object driven by a driving force, and more particularly to a method of controlling an industrial robot by suppressing vibration while a robot arm is running and when the robot arm is positioned in a stop position.
2. Description of the Prior Art
In recent years, in order to reduce the work load on workers in modern manufacturing plants, application of industrial robots has been on the increase. FIG. 1 of the accompanying drawings shows a conventional industrial robot of a biaxial control type in which the robot can rotate and can move vertically. As shown in FIG. 1, a finger portion 5 for gripping a workpiece is mounted on the distal end of a robot arm 4, which is driven by a servo motor 3 for rotational motion and a raising and lowering motion. The servo motor 3 is driven from a controller 1 via a servo amplifier 2.
If the above-mentioned robot is accelerated at a point a in FIG. 1 from a rest state to a predetermined speed and is then slowed down to assume a rest state at a point b, vibration would occur, particularly at points where acceleration varies largely, which might cause an error in controlling the robot so that it is necessary to wait for the next action until the vibration comes to an end. On many occasions, the points where this vibration occurs is start and end points of each of acceleration and slowdown. The vibration occurring during accelerating remains at the time of constant speed motion, which might cause an error in positioning the finger portion 5. The vibration at the time of constant speed motion would be a cause for nonuniform coating in, for example, coating work. In the presence of the vibration occurring during slowdown, after coming to rest, the robot has to wait for work such as assembling work, until the vibration comes to an end, thus resulting in a waste of time.
Studies have been made to suppress vibration of the robot; to this end, it is currently known to control vibration as disclosed in, for example, Japanese Patent Laid-Open Publications Nos. SHO 63-314606 and 63-314607. According to this apparatus, the vibration occurring at the distal end of a robot arm is detected by an acceleration sensor mounted on the robot arm end and this detection signal is back as a turbulent torque, thus adjusting the driving force to the occurred vibration.
In this prior art, however, damping control takes place after vibration has actually occurred so that, although the time from the occurrence of vibration to the end can be reduced, vibration cannot be eliminated. In other words, since control information to bring vibration to an end is obtained from the status of actual vibration, damping control cannot be performed until after vibration has occurred. It is therefore theoretically impossible to perfectly control vibration. Further, since vibration is damped following vibration of a relatively short period, this prior art control system requires a central processing unit (CPU) which enables high-speed computing. Accordingly the resultant control system is expensive and complicated.